Breath switch

ABSTRACT

An improved switching device for indicating the beginning and ending of the breath of a test subject. A flow through housing is provided with an inlet and first and second outlets. A flexible diaphragm is connected in series with the first outlet to serve as a check valve which opens during the high flow rate portion of a breath. A sensitive flow responsive member connected in series with the second outlet opens in response to the presence of gas flow during the low flow rate portion of the breath. A detecting circuit responsive to the position of the flow responsive member accurately signals the beginning and ending of a breath without interference by the check valve.

BACKGROUND OF THE INVENTION

The present invention relates to flow responsive switches and isdirected more particularly to a flow responsive switch for indicatingthe beginning and ending of the breath of a test subject.

In evaluating the physical condition of a test subject, usefulinformation can be derived by measuring the volume and composition ofthe test subject's exhaled breath. Instruments for performing thesemeasurements usually include gas analyzers for measuring at least theconcentration of oxygen and carbon dioxide in the exhaled breath.Measurements of this type may be performed under conditions which rangefrom a condition of rest to a condition of vigorous exercise. In orderto properly interpret the measurements, however, it is necessary to takeinto account the volume of the breath. This volume, in turn, is usuallymeasured indirectly by measuring the rate of flow and duration of eachbreath.

The rate of flow of a breath is conveniently measured by directing thebreath through a gas turbine which is arranged to generate a series ofpulses having a repetition rate that varies with the rate of gas flowtherethrough. Because of the non-linearity of gas turbines, the volumeof a breath can be determined from the number of pulses it produces onlyby taking into account the duration of a breath. The number of pulsesand the duration of that breath may, for example, be combined to producea pulses-per-second value which can then be combined with the knowncharacteristics of the turbine to yield the volume of that breath.

In making the above measurement it has been found that much of the errorthat occurs results not from the operation of the turbine, but ratherfrom the inaccurate determinaton of the duration of the breath. Anaccurate determination of the end of a breath has been found to beparticularly difficult because of the low rates of flow that areassociated therewith when the test subject is at rest.

Prior to the present invention, it was the practice to direct a flow ofbias gas through the turbine and thereby cause the latter to generate apredetermined minimum number of turbine pulses per unit time. With theuse of this approach the beginning of breath is taken as the time whenthe pulse "frequency" increases above its minimum value, and the endingof the breath is taken as the time when the pulse "frequency" returns tothat minimum value. Because the beginning of a breath is usuallycharacterized by a relatively sudden increase in the rate of gas flow,the time at which a breath begins can be determined relativelyaccurately in this way. Because, however, the ending of a breath ischaracterized by a low and slowly changing rate of flow, the time atwhich a breath ends cannot be determined accurately in this way. As aresult, the measured duration of a breath can be in error by severalpercent, causing a corresponding error in the measured volume of thebreath.

SUMMARY OF INVENTION

In accordance with the present invention there is provided an improvedbreath duration measuring device, hereinafter referred to as a breathswitch, which signals the beginning and ending of breath with improvedaccuracy and which does not require the use of a bias gas flow.

Generally speaking, the breath switch of the invention includes ahousing that provides first and second paths through which a breath maybe vented to the atmosphere. The first or main flow path conducts theflow of most of an exhaled breath through a check valve comprising aflexible diaphragm which is gently biased in its closed position.Because of this biasing, the valve will open only when the pressure ofan exhaled breath exceeds a predetermined minimum value. Once open,however, the check valve provides little fluidic resistance to the flowof a breath.

The second flow path through the housing bypasses the check valve andthereby allows an exhaled breath to continue to flow after the checkvalve closes. This second flow path is provided with a flow responsivemember which is highly sensitive to the flow of breath. The position ofthis member is used to signal the beginning and ending of a breath andthereby provide breath duration data of greatly improved accuracy.

In the preferred embodiment, the flow responsive member is a flexiblemetal foil which is connected across the second flow path and which isarranged to inerrupt or not interrupt a beam of light which isestablished by an associated optoelectronic detector circuit. Because ofthe flexibility of the flow responsive member it is able to clearlysignal the beginning and ending of breath, even at the extremely lowflow rates that may be associated with those conditions. The detectorcircuit is therefore able to provide a sensitive and unambiguousindication of the duration of a breath.

Together, the above features allow the breath switch of the invention tohave the sensitivity necessary to accurately measure the beginning andending of breath at low flow rates, and yet be able to deal with thehigh breath flow rates that are associated with the taking ofmeasurements under exercise conditions.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of the breath switch of the invention, shown withthe upper part of its housing removed,

FIG. 2 is a cross-sectional view of the switch of FIG. 1 taken along thesection line 2--2,

FIG. 3 is a cross-sectional view of the switch of FIG. 1 taken along thesection line 3--3,

FIGS. 4, 5, 6 and 7 together comprise an exploded view of the upper partof the housing of the breath switch of the invention,

FIG. 4a shows a bottom view of the element shown in FIG. 4,

FIG. 6a shows a top view of the element shown in FIG. 6,

FIG. 7 shows a side view of a retaining ring that holds the upper andlower parts of the housing together, and

FIG. 8 shows the appearance of the fully assembled breath switch.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1, 2 and 3 include a top and two cross-sectional views of thelower half of the breath switch of the invention. This lower half fitstogether with an upper half, which is shown in exploded form in FIGS. 4,5, 6 and 7 to form a complete breath switch assembly. The completeassembly, the appearance of which is shown in FIG. 8, is adapted to beconnected in series with the outlet of a breath gas analyzer (not shown)and to apply to the conductors of a cable 8 electrical signals whichindicate beginning and ending of breath flow.

As shown in FIGS. 1, 2 and 3, the lower half of the breath switchincludes a first housing section 10a. This housing section, which ispreferably made of a suitable injection molded plastic, includes a gasinlet 12 which is adapted to be connected in series with the outlet ofthe breath gas analyzer. Inlet 12 preferably includes a tapered interiorsurface 11 which can fit over and be supported by the outlet of a gasturbine (not shown) which measures the rate of gas flow through theanalyzer.

Housing section 10a is provided with an internal mounting shoulder 14which is clearly visible in FIG. 3. This shoulder serves as a stop for asecond housing section 10b, shown in FIGS. 4 and 8, which fits intohousing section 10a to form a closed housing. Housing section 10b isheld in place with respect to housing section 10a by a suitableretaining ring, shown in FIG. 7, which may be attached to housingsection 10a by suitable screws (not shown). By closing the upper end ofhousing section 10a, housing section 10b forces the gas that flows intoinlet 12 to flow out of one or the other or both of two outlets. Thefirst outlet, 16, shown in FIGS. 4 and 4a, is relatively large andoccupies the central part of housing section 10b. This outlet serves toconduct the greater part of the flow of an exhaled breath. The secondoutlet, 18, has a relatively small area and is located to one side offirst outlet 16. Outlet 18 serves to conduct the lesser part of the flowof an exhaled breath, except under those conditions in which firstoutlet 16 is blocked, as will be explained more fully hereafter. Thesefirst and second outlets are associated with two different respectiveflow paths through the interior of the breath switch.

Referring again to FIG. 1, there is shown a partition or wall 20 whichserves to divide the interior of the housing into a first chamber 22 anda second chamber 24. First chamber 22, which is connected between inlet12 and first outlet 16, provides a first path for the flow of breaththrough the housing. Second chamber 24, which is connected between inlet12 and second outlet 18, through chamber 22, provides a second flow pathfor the flow of gas through the housing. During operation, breath willflow through the second or through the first and second of these paths,depending upon the rate of flow or force of that breath.

Partition 20 is provided with an elongated opening or slot 26 in whichis located a flexible flow responsive member or flapper 28 that isattached to partition 20 by a suitable mounting screw 30 that is bestseen in FIGS. 1 and 3. This screw preferably passes through member 28and screws into a threaded hole in a mounting block 32 to hold member 28against partition 20.

When no gas flows through the breath switch, member 28 assumes a rest orunactuated position as shown in FIGS. 1 through 3. In this positionmember 28 substantially blocks opening 26, except for clearance spaces,that are too small to be clearly shown, which lie along the edges ofmember 28. When, on the other hand, gas does flow through the breathswitch, member 28 is displaced from the position shown in FIG. 1 toassume its actuated position. In the latter position member 28 is flexedor bent so that the tip 28a thereof moves out of opening 26 and providesa clear path for the flow of gas between chambers 22 and 24. Thus, flowresponsive member 28 assumes one of two positions or states dependingupon the presence or absence of breath flow at inlet 12.

In order to make member 28 sensitive to very low rates of breath flow,member 28 is preferably made of a material, such as a metal foil, whichis sufficiently flexible to bend readily when a pressure differenceappears across it, but which is sufficiently resilient to return to itsoriginal position when the pressure difference is removed. One materialwhich has been found to have these properties is a one mil piece ofaustenitic stainless steel. Other suitable materials will be apparent tothose skilled in the art.

In the preferred embodiment, partition 20 is provided with a projectingchannel 38 that surrounds at least the movable portion of member 28. Theeffect of projecting channel section 38 is to reduce the amount of gasthat flows past the upper and lower edges of member 28 and thereby causethe gas in the second flow path to concentrate at the end 28a thereof.Since the end of member 28 is the most easily moved part thereof, thisassures that even a small difference between the pressures in chambers22 and 24 produces a relatively large movement of end 28a of member 28.

The sensitivity of the flow responsive member may be further enhanced byincluding in housing section 10b an elongated tapered passage 19 betweenchamber 22 and outlet 16. Because the wall of this passage defines atapered surface 21 which projects downwardly to a point above inlet 12,as shown in FIG. 8, it tends to deflect a part of the entering gasstream toward the periphery of housing section 10a. This outwarddeflection of the entering gas stream has in some cases been found toimprove the dynamic response of member 28 by concentrating the flow ofgas thereagainst. Accordingly, a flow deflecting surface such as 21which is located adjacent to slot 26 is regarded as a desirable but notessential feature of the present invention.

During the taking of measurements, it is important that gas not be drawninto the gas analyzer, through outlet 16, when the test subject isinhaling. This is because such an inward flow of gas would mixatmospheric gas with the previously exhaled breath gases that are stillin the analyzer and thereby introduce an error into the gasconcentration readings. In order to prevent this from occurring, thebreath switch is provided with a check valve 39 which fits over outlet16 in housing section 10b. In the preferred embodiment check valve 39includes a flexible elastomeric diaphragm 40 which is shown in FIGS. 6and 6a. As shown in FIG. 6, diaphragm 40 is connected to a circular base42 by strips 44 which are formed integrally with diaphragm 40 and base42. These connecting strips serve as springs to pull diaphragm 40downwardly, toward base 42, to close the check valve when no offsettingforce tends to move diaphragm 40 in the opposite direction.

During assembly, check valve 39 is pulled down over an annular member46, shown in FIG. 5, until the inwardly projecting lip 42a of base 42slips over and locks into place around an outwardly projecting ridge 46bon member 46. Member 46 is then pushed down over the raised rim 48 ofoutlet 16 where it is held by the tightness of the fit and effectivelybecomes a part of housing section 10b. When check valve 39 is properlypositioned with respect to membrane 46, diaphragm 40 will rest on theupper edge 46a of member 46 which acts as a valve seat, will be pushedagainst that valve seat with the desired force. This force is such thatdiaphragm 40 will lift off of valve seat 46a to open the check valvewhen the pressure of an exhaled breath exceeds a threshold value that islow enough to prevent the application of objectionable back pressure tothe test subject. Diaphragm 40 will therefore tend to remain closed atthe beginning and ending of each breath, but will open to provide a lowresistance path for breath flow during the high flow rate portionthereof.

Because of the elasticity of diaphragm 40, it tends to vibrate orflutter at low breath flow rates. This flutter is undesirable because itcan cause unintended fluctuations in the position of flow responsivemember 28. In order to eliminate these fluctuations, a flexibleelastomeric vibration damping member 50 is attached to the upper surfaceof diaphragm 40 by a drop 52 of a suitable glue. Because it is sizeddifferently than diaphragm 40, damping member 50 tends to vibrate at adifferent frequency. In the preferred embodiment damping member 52 is sosized relative to diaphram 40 that its vibrations destructivelyinterfere with those of diaphragm 40 and thereby stabilize the operationof valve 39. This stabilization has been found to significantly improvethe ability of the breath switch to precisely detect the beginning andending of a breath.

In accordance with the present invention, the flexibility of member 28is such that it will assume its actuated position when the pressure inchamber 22 is too low to open check valve 39. This same flexibilitypermits member 28 to remain in its actuated position after the pressurein chamber 22 becomes too low to hold check valve 39 open. Since, suchlow pressure conditions exist at the beginning and ending of a breath,changes in the position of member 28 signal the beginning and ending ofa breath with much greater sensitivity than changes in the position ofcheck valve 39.

During the high pressure, high flow rate portion of a breath, checkvalve 39 will open to provide a low resistance to the flow of breaththrough the first flow path and opening 16. This low resistance patheffectively bypasses the flow of breath through the second flow path andoutlet 18, and thereby prevents the application of objectionable backpressure to the test subject. Since, however, the pressure that isnecessary to hold the check valve open is high enough to keep member 28from returning to its rest position, member 28 will remain in itsactuated position during the high flow rate portion of an exhaledbreath. Thus, member 28 will remain in its actuated position during boththe high and low flow rate portions of a breath.

To the end that the position of flow responsive member 28 may becommunicated to the instrument with which the breath switch is used, thebreath switch is provided with a detecting circuit 60 which is shown inFIGS. 1 and 3. In the preferred embodiment detecting circuit 60 is anoptoelectronic circuit including a light emitting device such as an LED62 which is mounted on a first circuit board 64, and a light responsivedevice such as a phototransistor 66 which is mounted on a second circuitboard 68. LED 62 is supplied with operating power through conductors 8aand b of cable 8. Similarly, phototransistor 66 is supplied withoperating power through conductors 8a and c of cable 8. The connectionsbetween the conductors 8a through c and devices 62 and 66 are made viaconductive traces which are deposited on the two sides of circuit boards64 and 68 in a manner well known to those skilled in the art.

In order that the transmission of light from LED 62 to phototransistor66 may be controlled in accordance with the position of member 28,member 28 is provided with a light interrupting vane 28b which projectsthrough slot 26. When member 28 is in its rest position, vane 28b blocksthe transmission of light from LED 62 to phototransistor 66. Under thiscondition, only a small current will flow in conductors 8a and c. Wheneven a gentle flow of breath is present, however, member 28 will assumeits actuated position to remove vane 28b from between LED 62 andphototransistor 66 and thereby permit light to be transmittedtherebetween. Under this condition, a sizable current will flow inconductors 8a and c. It will therefore be seen that the current inconductors 8a and c is direct and sensitive indication of the beginningand ending, and therefore the duration of a breath. It will beunderstood that other detecting circuits, such as those using halleffect devices, may be substituted for the optoelectronic circuitryshown in FIG. 3 without significantly affecting the operation thereof.

During the time that a test subject is breathing in, the low pressure inchamber 22 tends to draw member 28 inwardly through slot 26. Becausethis movement can cause small amounts of air to be drawn in inwardlythrough outlet 18, it is desirable to keep such movement to a minimum.One convenient way of limiting this inward movement is to positioncircuit boards 64 and 68 so that the edges thereof act as stops formember 28. By limiting the movement of member 28 these stops causemember 28 to act as a check valve to block the flow of air from outlet18 into chamber 22. Other ways of blocking the inward flow of airthrough outlet 18 will be apparent to those skilled in the art.

To the end that member 28 may be adjusted so that it assumes the restposition shown in FIG. 1, there is provided an adjustment screw 70. Thisscrew, which passes through a threaded hole in mounting block 32, pushesagainst member 28 near the non-movable end thereof. By turning screw 70in one direction or the other the rest position of member 28 may bemoved further into or out of slot 26. Since adjustment screw 70 can pushbut not pull member 28, it is desirable that any natural curvature inmember 28 be such that the latter pushes against screw 70. Externalaccess to adjusting screw 70 is conveniently provided via an access hole72 through housing section 10a. Because this access hole is fluidicallyin parallel with outlet 18, it does not interfere with the operation ofblocking member 28.

In order to make the breath switch easy to produce and assemble,partition 20 is preferably formed as a separate piece and not as a partof housing sections 10a or 10b. This separateness makes it possible toattach to partition 20 all of the mechanical and electrical devices thatare mounted thereon before partition 20 is inserted into the housing.This separateness also makes it necessary, however, to provide retainingstructures which can receive and hold the edges of partition 20 andthereby assure that partition 20 is held in the proper position withinthe housing. In housing section 10a the retaining structures includevertical retaining strips 34a and 34b and horizontal retaining strips35a and 35b, all of which project inwardly from the inner surface ofhousing section 10a. In housing section 10b the retaining structuresinclude retaining strips 36a and 36b which project inwardly from theinner surface of that housing section. By slipping into the spacesbetween these retaining strips, partition 20 is held firmly in thedesired position within the housing. In the latter position, partition20 substantially prevents gas from flowing into chamber 24 around theedges thereof.

While chamber 24 is shown as being connected to inlet 12 through chamber22, chamber 24 may also be directly connected to inlet 12. Inlet 12 may,for example, be partitioned into a first large inlet section which leadsdirectly to chamber 22 and a second, smaller inlet section which leadsdirectly to chamber 24. Because, however, embodiments of the latter typeprovide no advantage over the embodiment of FIGS. 1 through 6, andcomplicate the internal structure of the housing, they are not regardedas preferred embodiments of the present invention. They are neverthelessconsidered to be within the scope of the present invention.

In view of the foregoing it will be seen that a breath switchconstructed in accordance with the present invention provides a numberof advantages over previously used breath duration measuring devices. Inparticular the breath switch of the invention can detect the beginningand ending of a breath with greater accuracy and sensitivity thanpreviously used devices. In addition the breath switch of the inventionmakes possible the elimination of the cost that is associated with theuse of a bias gas source. Finally, the breath switch of the inventionincludes an improved vibration damped check valve which eliminates thepotential gas flow instabilities that are associated with low gas flowrates.

What is claimed is:
 1. A switch for indicating the beginning and endingof a pulsatile flow of gas comprising:(a) a housing having an inlet andfirst and second outlets, (b) means defining a first flow path for theflow of gas between the inlet and the first outlet, wherein the firstflow path includes a first chamber which is connected to the inlet andto the first outlet, (c) means defining a second flow path for the flowof gas between the inlet and the second outlet, wherein the second flowpath includes a second chamber which is connected to the first chamberand to the second outlet, (d) a check valve connected in series with thefirst flow path, (e) a flow responsive member located between the firstand second chambers and connected in series with the second flow pathand adapted to move in response to the flow of gas through the secondflow path, and (f) means for indicating the duration of said pulsatileflow in response to the position of said flow responsive member whereinsaid indicating means is triggered by the flow responsive member inaccordance with the flow of gas in the second flow path.
 2. The switchof claim 1 including means for deflecting gas flowing into the inlettoward the flow responsive member.
 3. The switch of claim 1 in which thefirst outlet is large relative to the second outlet.
 4. The switch ofclaim 1 in which the check valve comprises an elastomeric diaphragm. 5.The switch of claim 1 in which the flow responsive member has a flexiblebody which flexes before the pressure of the gas at the inlet issufficient to open the check valve.
 6. A switch for indicating thebeginning and ending of a pulsatile flow of gas comprising:(a) a housinghaving an inlet and first and second outlets, (b) means defining a firstflow path for the flow of gas between the inlet and the first outlet,(c) means defining a second flow path for the flow of gas between theinlet and the second outlet, (d) a check valve connected in series withthe first flow path, (e) a flow responsive member connected in serieswith the second flow path and adapted to move in response to the flow ofgas through the second flow path, wherein the flow responsive member hasa flexible body which flexes before the pressure of the gas at the inletis sufficient to open the check valve and remains flexed after thepressure of the gas at the inlet is sufficient to open the check valve,and (f) means for indicating the position of said flow responsivemember.
 7. A switch for indicating the beginning and ending of apulsatile flow of gas comprising:(a) a housing having an inlet and firstand second outlets, (b) means defining a first flow path for the flow ofgas between the inlet and the first outlet, (c) means defining a secondflow path for the flow of gas between the inlet and the second outlet,(d) a check valve connected in series with the first flow path, (e) aflow responsive member connected in series with the second flow path andadapted to move in response to the flow of gas through the second flowpath and adapted to prevent air from being drawin into the housingthrough the second outlet, and (f) means for indicating the position ofsaid flow responsive member.
 8. A switch for measuring the duration of apulsatile flow of gas comprising:(a) a housing having an inlet and firstand second outlets, (b) a first housing chamber open to the inlet andthe first outlet, (c) a second housing chamber open to the secondoutlet, (d) a check valve connected in a series with the first outlet,(e) means defining a passage between the first and second chambers, (f)a flow responsive member located in said passage, said flow responsivemembers being adapted to assume a rest position when approximately nogas flows into the inlet and to assume an actuated position when gasdoes flow into the inlet, and (g) means for detecting the position ofthe flow responsive member, wherein the detecting means includes anemitting device and a receiving device, and wherein the flow responsivemember interrupts a transmission between the emitting and receivingdevices.
 9. The switch of claim 8 in which the emitting and receivingdevices are supported on said defining means.
 10. A switch for measuringthe duration of a pulsatile flow of gas comprising:(a) a housing havingan inlet and first and second outlets, (b) a first housing chamber opento the inlet and the first outlet, (c) a second housing chamber open tothe second outlet, (d) a check valve connected in a series with thefirst outlet, (e) means defining a passage between the first and secondchambers, (f) a flow responsive member located in said passage, saidflow responsive member being adapted to assume a rest position whenapproximately no gas flows into the inlet and to assume an actuatedposition when gas does flow into the inlet and to block the flow of airinto the housing though the second outlet, and (g) means for detectingthe position of the flow responsive member.
 11. A switch for measuringthe duration of a pulsatile flow of gas comprising:(a) a housing havingan inlet and first and second outlets, (b) a first housing chamber opento the inlet and the first outlet, (c) a second housing chamber open tothe second outlet, (d) a check valve connected in a series with thefirst outlet, (e) means defining a passage between the first and secondchambers, wherein the defining means comprises a wall that separates thefirst and second chambers and in which the passage comprises an openingin said wall, and (f) a flow responsive member located in said passage,said flow responsive member having a flexible body which is attached tothe wall in the vicinity of the opening and being adapted to assume arest position when approximately no gas flows into the inlet and toassume an acutated position when gas does flow into the inlet, (g) meansfor detecting the position of the flow responsive member, and (h) meansfor adjusting the rest position of the flexible body.
 12. A switch formeasuring the duration of a pulsatile flow of gas comprising:(a) ahousing having an inlet and first and second outlets, (b) a firsthousing chamber open to the inlet and the first outlet, (c) a secondhousing chamber open to the second outlet, (d) a check valve connectedin a series with the first outlet, (e) means defining a passage betweenthe first and second chambers, (f) a flow responsive member located insaid passage, said flow responsive member being adapted to assume a restposition when approximately no gas flows into the inlet and to assume anactuated position when gas does flow into the inlet and to remain in theactuated position when the check valve is open, and (g) means fordetecting the position of the flow responsive member.
 13. The switch ofclaim 12 in which the check valve comprises a flexible diaphragm. 14.The switch of claim No. 12 in which the first outlet is large relativeto the second outlet.
 15. The switch of claim No. 12 including means fordeflecting gas flowing into the inlet toward the flow responsive member.